The invention relates to a direct injection spark ignition internal combustion engine with one injector per cylinder for injecting fuel into a combustion chamber delimited by a piston, the piston having a recess formed therein and the injector having injection ports distributed over the circumference for injecting fuel into the recess in the piston.
In direct-injection internal combustion engines, the fuel/air mixture which is to be combusted in order to drive the piston and which is composed of fuel injected directly into the combustion chamber to mix with combustion air which is supplied to the cylinders separately through inlet ducts, is formed in the combustion chamber of each cylinder. In the case of spark-ignition internal combustion engines the fuel/air mixture has to be ignited by the ignition spark of a spark plug which projects into the combustion chamber and an ignitable mixture therefore has to be made available at the electrodes of the spark plug. In a stratified charge operating mode of the engine, it is possible, by late injection of fuel during the working cycle of the respective cylinder, to form a stratified mixture cloud with an ignitable fuel concentration when the overall mixture in the entire volume of the combustion chamber is lean. The operation with a stratified mixture leads to a reduction in the fuel consumption and the emission of pollutants by the internal combustion engine. In order to ensure that the mixture can be ignited in the region of the spark plug, in particular in the stratified charge operating mode, the fuel is usually injected into the combustion chamber in a conical jet, forming a conical cloud of fuel and combustion air. In the case of what is referred to as a jet-controlled combustion method, the formation of the mixture depends greatly on the quality of the ignitable mixture of the cloud of fuel in the region of the spark plug. In order to ensure a stable formation of mixture and ignition of the mixture in the case of a jet-controlled combustion method by means of a satisfactorily configured, conical cloud of fuel, so-called multi-hole injectors are provided which have a plurality of injection holes distributed over the circumference of the injector.
DE 198 04 463 A1 discloses such a multi-hole injector in which at least one row of injection holes distributed over the circumference of the injection nozzle is provided in order to implement a jet-controlled combustion method by forming the cloud of mixture by selective injection of fuel via the injection holes.
The geometrical shape of the combustion chamber is influenced by a piston recess which is formed in the piston head of the piston. The piston recess is located here approximately opposite the injection nozzle and represents the residual volume of the combustion chamber, in which the injected fuel is located, in the region of the top dead center of the movement of the piston. DE 199 22 964 A1 discloses a geometric configuration of the piston recess for improving the mixture formation with fuel which is injected in a conical shape. In particular in the case of auto-ignition diesel internal combustion engines, the geometric shape of the combustion chamber has a decisive influence on the quality of the mixture formation. The known configuration of the combustion chamber has in this context what is referred to as an omega piston recess in which the fuel is directed by a central elevation on the bottom of the piston recess into the outer edge regions of the circular piston recess in order to improve the formation of the fuel/air mixture.
When multi-hole injectors are used for the internal formation of the mixture in direct-injection spark-ignition engines the fuel is injected as late as possible during the compression stroke of the piston in order to bring about the greatest possible degree of stratification of the mixture cloud. During operation of such internal combustion engines, it has been repeatedly detected that the emission of pollutants is undesirably high and the surface of the piston also tends to coke up. Such phenomena are due to wetting of the surface of the piston with the injected fuel, which wetting is unavoidable despite the internal pressure of the cylinder which is particularly high at the injection time because the distance between the injector and piston head surface is very short at that point.
It is the object of the present invention to provide an internal combustion engine wherein wetting of the piston with fuel during the injection is prevented even during operation with late injection of fuel.